This application claims the benefit of Japanese Patent application No. 2000-347095 which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an immersion microscope objective lens, and particularly, to an objective lens which is suitable for an immersion microscope for observing an optical path between the objective lens and an object to be observed in a state that the optical path is filled with liquid such as water or oil.
2. Related Background Art
In an immersion microscope objective lens of high magnification, a piano-convex lens having a relatively low refractive index is embedded in a lens at the tip end in order to correct a curvature of field and a negative refracting power is given to a cemented surface by means of a difference of refractive index between the two lenses, thereby reducing a Petzval sum. Further, a pair of concave surfaces facing each other are disposed at positions with a small diameter of light flux. The Petzval sum is decreased by the strong refracting power of these concave surfaces.
That is, an arrangement which combines the reduction of a Petzval sum by using an embedded lens with that by using a so-called Gauss type lens is known as a publicly known art. An apochromat type objective lens of high magnification which serves as such an immersion microscope objective lens is disclosed in, for example, the Japanese Patent Application Laid-Open No. 2000-35541.
Generally, when a Petzval sum is to be reduced by an embedded plano-convex lens at the tip end, it is advantageous that a meniscus lens into which the plano-convex lens is embedded should have as great thickness as possible and should be formed of an optical material having a higher refractive index. In this case, however, the spherical aberration of a color developed on a convex surface with a strong curvature facing the image side of the meniscus lens in which the plano-convex lens is embedded is increased.
The above phenomenon is caused by the fact that a dispersion (a difference of refractive index by a wavelength) tends to be relatively great when the refractive index of an optical material is high, and the fact that, when the meniscus lens is made thick, the height h of a light flux on the refractive surface on the air side thereof becomes great. Also in the conventional immersion microscope objective lens disclosed in the above patent application, aberrations are not sufficiently corrected and, particularly, a high-order component of a spherical aberration (a different of spherical aberration by a wavelength) of a color remains.
The present invention has been contrived taking the above problems into consideration, and an object thereof is to provide an immersion microscope objective lens which has a high magnification and a high numerical aperture, with excellently corrected aberrations over the wide field of view, and has excellent imaging performance.
In order to solve the above problems, according to the present invention, there is provided an immersion microscope objective lens comprising a first lens group G1 having a positive refracting power, a second lens group G2 having a positive refracting power and a third lens group G3 having a negative refracting power in the order from the object side, characterized in that:
the first lens group G1 has a first cemented lens comprising a plano-convex lens with the flat surface facing the object side and a meniscus lens with the convex surface facing the image side cemented together, a positive lens with the surface having a larger refractive index facing the image side, and a second cemented lens comprising a negative lens and a positive lens cemented together;
the second lens group G2 has a cemented lens comprising a negative lens and a positive lens cemented together; and
the refractive index n12 of the meniscus lens in the first lens group G1 with respect to the d-line satisfies the following condition:
n12 greater than 1.9. 
According to a preferred embodiment of the present invention, the following condition is satisfied:
5 less than |f31|/F less than 25, 
where the focal length of the negative lens for constituting the second cemented lens in the first lens group G1 is f31, and the focal length of the entire objective lens system is F.
Also according to a preferred embodiment of the present invention, the second lens group G2 has a third cemented lens composed of a negative lens and a positive lens cemented together, a fourth cemented lens comprising a negative lens and a positive lens cemented together, and a fifth cemented lens comprising a negative meniscus lens, a biconvex lens, and a biconcave lens cemented together, in the order from the object side.
Further, according to a preferred embodiment of the present invention, the third lens group G3 has a sixth cemented lens comprising a positive lens with the convex surface facing the object side and a negative lens with the concave surface facing the image side cemented together, and a seventh cemented lens comprising a negative lens with the concave surface facing the object side and a positive lens with the convex surface facing the image side cemented together, in the order from the object side; and the following conditions are satisfied:
n71 greater than 1.49; and 
n82 greater than 1.6, 
where the refractive index of the positive lens in the sixth cemented lens with respect to the d-line is n71 and the refractive index of the positive lens in the seventh cemented lens with respect to the d-line is n82.
Also, according to a preferred embodiment of the present invention, the refractive index n11 of the plano-convex lens in the first lens group G1 with respect to the d-line satisfies the following condition:
n11 greater than 1.55. 
In an immersion microscope objective lens according to a representative embodiment of the present invention, the first lens group G1 consists of three lens components. The first lens component is a so-called embedded lens component, which is the first cemented lens comprising a plano-convex lens with the flat surface facing the object side, and a meniscus lens in which this plano-convex lens is embedded cemented together.
In this case, due to an arrangement that the plano-convex lens has substantially the same refractive index as that of the used liquid (immersion liquid), little spherical aberration is generated advantageously on an interface between the liquid and the plano-convex lens. Further, since a strong negative refracting power is applied onto a cemented surface with the meniscus lens in which the plano-convex lens is embedded, a Petzval sum is reduced.
A light flux having passing through the first lens component is given a moderate refractive action by the second lens component which is a single lens having a positive refracting power. On this occasion, an angle of deviation of a marginal light beam is arranged to be as small as possible due to the arrangement that the positive lens for constituting the second lens component has the surface having a greater refracting power facing the image side.
The third lens component is adapted to correct a longitudinal chromatic aberration and a spherical aberration of a color generated on the convex surface having a strong curvature on the image side in the first and second lens components by the use of the second cemented lens which is constituted by the negative lens and the positive lens having approximately the same refractive indexes and different dispersions cemented together.
On the other hand, in the second lens group G2, a light emitted from the first lens group G1 is converted to a convergent light and a cemented lens consisting of a negative lens and a positive lens cemented together is disposed, thereby correcting a spherical aberration and a longitudinal chromatic aberration. In this case, it is the most preferable in terms of the aberration correction mentioned above to form the second lens group G2 of the two sets of cemented lenses consisting of negative lenses and positive lenses cemented together, that is, the third cemented lens and the fourth cemented lens, and the fifth cemented lens which consists of the negative meniscus lens, the biconvex lens and the biconcave lens.
In the third lens group G3, a longitudinal chromatic aberration and a lateral chromatic aberration are corrected in an well-balanced manner while a Petzval sum is reduced. In this case, when the lens group is arranged in a so-called Gauss type structure, a more excellent correction effect of the Petzval sum, and a more excellent correction effect of a curvature of the field can be expected in its turn.
The conditions of the present invention will be fully described below. According to the present invention, the refractive index n12 of the meniscus lens for constituting the first cemented lens in the first lens group G1, that is, the meniscus lens in which the plano-convex lens is embedded, with respect to the d-line satisfies the following condition (1):
N12 greater than 1.9xe2x80x83xe2x80x83(1). 
The condition (1) defines an appropriate range for the refractive index n12 of the meniscus lens in which the plano-convex lens is embedded. Below the lower limit of the condition (1), a sufficient negative refracting power can not be obtained on the cemented surface between the plano-convex lens and the meniscus lens. As a result, the Petzval sum can not be sufficiently reduced, thereby deteriorating the flatness of the image surface. In order to secure a more excellent imaging performance while satisfactorily correcting the curvature of field up to the visual field No. 25, it is desirable to set the lower limit for the condition (1) at 2.0.
Also, according to the present invention, it is preferable to satisfy the following condition (2):
5 less than |f31|/F less than 25xe2x80x83xe2x80x83(2), 
where f31 is the focal length of the negative lens for constituting the second cemented lens in the first lens group G1, and F is the focal length of the entire objective lens system.
The condition (2) defines a condition for satisfactorily correcting a longitudinal chromatic aberration and a spherical aberration of color. Beyond the range for the condition (2), it becomes undesirably difficult to correct not only the chromatic aberration, but also the spherical aberration. Note that it is preferable to set the upper limit for the condition (2) at 21 and the lower limit at 7, in order to secure a more excellent imaging performance while maintaining the numerical aperture NA of 1.4 or more.
Also, according to the present invention, it is preferable that third lens group G3 should have the sixth cemented lens which comprises the positive lens with the convex surface facing the object side and the negative lens with the concave surface facing the image side cemented together, in the order from the object side, and the seventh cemented lens which comprises the negative lens with the concave surface facing the object side and the positive lens with the convex surface facing the image side cemented together, in the order from the object side, and should satisfy the following conditions (3) and (4):
n71 greater than 1.49xe2x80x83xe2x80x83(3); and 
n82 greater than 1.6xe2x80x83xe2x80x83(4), 
where n71 is the refractive index of the positive lens in the sixth cemented lens group with respect to the d-line, and n82 is the refractive index of the positive lens in the seventh cemented lens with respect to the d-line.
The condition (3) defines an appropriate range for the refractive index n71 of the positive lens in the sixth cemented lens in the third lens group G3 which is arranged of the Gauss type. Beyond the range for the condition (3), the radius of curvature of the surface on the air side (surface on the object side) of the positive lens in the sixth cemented lens becomes too small to sufficiently reduce the Petzval sum, which will undesirably deteriorate a coma. Note that, in order to further secure an excellent imaging performance while satisfactorily correcting the curvature of field, it is desirable to set the lower limit for the condition (3) at 1.56.
The condition (4) defines an appropriate range for the refractive index n82 of the positive lens in the seventh cemented lens in the third lens group G3 arranged to be of the Gauss type. Beyond the range for the condition (4), the radius of curvature of the surface on the air side (surface on the image side) of the positive lens in the seventh cemented lens becomes too small to sufficiently reduce the Petzval sum, which will undesirably deteriorate a coma. Note that, in order to further secure an excellent imaging performance while satisfactorily correcting the curvature of field, it is desirable to set the lower limit for the condition (4) at 1.80.
Also, according to the present invention, when the numerical aperture of the objective lens exceeds 1.4, it is preferable that the refractive index n11 of the plano-convex lens in the first lens group G1 with respect to the d-line satisfies the following condition (5):
n11 greater than 1.55xe2x80x83xe2x80x83(5). 
When liquid having a comparatively high refractive index such as oil having a refractive index of 1.55 or higher is used to secure a high numerical aperture, it is possible to satisfactorily suppress a spherical aberration which may be generated on an interface between the liquid and the plano-convex lens.